Someone's trying to make a buck off Ozzy Osbourne's name and his long-running concert festival ... according to a new lawsuit.

Ozzy's company's unleashing their lawyers on a Central Park music event called OZY Fest -- short for OZY Fusion Fest -- which went down this past July and featured acts like Jason Derulo and Talib Kweli. Despite the name ... rock's Prince of Darkness, Ozzy, was NOT part of the lineup.

According to the suit, Ozzfest -- which has been a thing for more than 20 years -- fired off a cease and desist letter, warning that the event, and its logo, were confusing music fans. OZY Fest claimed it wasn't a concert, and more of a "media-focused event" -- whatever that means.

Team Ozzy's calling BS, especially because OZY Fest's own website describes it as an "all-day party that brings together incredible music." So, yeah ... just like Ozzy's incredibly popular and already established festival.

Ozzfest is suing to prevent an encore use of the name, and for profits from that July show.